BinModifier default value or null

This might not be an issue at all, but to record different behaviour between our parser and prometheus default.

For this query statement, the output of prometheus 3.0 `parse_query` of `process_virtual_memory_bytes / process_cpu_seconds_total` is like:

```json
{
"data": {
"bool": false,
"lhs": {
"matchers": [
{
"name": "__name__",
"type": "=",
"value": "process_virtual_memory_bytes"
}
],
"name": "process_virtual_memory_bytes",
"offset": 0,
"startOrEnd": null,
"timestamp": null,
"type": "vectorSelector"
},
"matching": {
"card": "one-to-one",
"include": [],
"labels": [],
"on": false
},
"op": "/",
"rhs": {
"matchers": [
{
"name": "__name__",
"type": "=",
"value": "process_cpu_seconds_total"
}
],
"name": "process_cpu_seconds_total",
"offset": 0,
"startOrEnd": null,
"timestamp": null,
"type": "vectorSelector"
},
"type": "binaryExpr"
}
}
```

The `matching` part

```
"matching": {
"card": "one-to-one",
"include": [],
"labels": [],
"on": false
},
```

In our implementation, `BinModifier` is set to None in this case.

There is a case when `matching` is set to `Null` in prometheus: `1 / process_cpu_seconds_total`

```json
{
"data": {
"bool": false,
"lhs": {
"type": "numberLiteral",
"val": "1"
},
"matching": null,
"op": "/",
"rhs": {
"matchers": [
{
"name": "__name__",
"type": "=",
"value": "process_cpu_seconds_total"
}
],
"name": "process_cpu_seconds_total",
"offset": 0,
"startOrEnd": null,
"timestamp": null,
"type": "vectorSelector"
},
"type": "binaryExpr"
}
}
```

## Conclusion

For binary expression, prometheus only set `matching` to null when one of the argument is literal, when both are vectorSelector, prometheus generates default `matching: {"card": "one-to-one"}`.

In our implementation, `BinModifier` is always set to null if no modifier provided.